Fruits are ripened from the gaseous plant hormone ethylene, which is best triggered using high temperatures or by wounding the flesh of fruits. Plants produce their own ethylene, and when the gas is trapped, such as in a closed paper bag with the fruit, the fruit ripens more hastily.
To ripen fruit with a paper bag, store it at room temperature, out of direct sunlight and in a dry location. This method is suitable for fruits like peaches, plums, pears, mangoes and kiwi fruit, and it works for bananas, nectarines, figs and avocados. The process can be sped up by creating a gash in one of the fruits. To prevent over-ripening, check the fruit daily. Once fruits ripen, remove them from the bag and place them on the counter, or refrigerate them for several more days if they are not to be eaten with a day or two.
Refrigeration stops the ripening of many fruits. While this can offer a longer shelf life for many fruits, it can also prevent the fruits from becoming sweet. Before ripening, many fruits should not be refrigerated, because this process can inactivate enzymes that are necessary for complete ripening and cause low-temperature injury to the fruits.